DESCRIPTION

This is a very simple and fast io format for PerlDL. The disk data consists of two files, a header metadata file in ASCII and a binary file consisting simply of consecutive bytes, shorts or whatever.

It is hoped that this will not only make for a simple PerlDL module for saving and retrieving these files but also make it easy for other programs to use these files.

The format of the ASCII header is simply

<typeid>
<ndims>
<dim0> <dim1> ...

You should probably stick with the default header name. You may want to specify your own header, however, such as when you have a large collection of data files with identical dimensions and data types. Under these circumstances, simply specify the Header option in the options hash.

The binary files are in general NOT interchangeable between different architectures since the binary file is simply dumped from the memory region of the piddle. This is what makes the approach efficient.

It is also possible to mmap the file which can give a large speedup in certain situations as well as save a lot of memory by using a disk file as virtual memory. When a file is mapped, parts of it are read only as they are accessed in the memory (or as the kernel decides: if you are reading the pages in order, it may well preread some for you).

Note that memory savings and copy-on-write are operating-system dependent - see Core.xs and your operating system documentation for exact semantics of whatever. Basically, if you write to a mmapped file without ReadOnly, the change will be reflected in the file immediately. ReadOnly doesn't really make it impossible to write to the piddle but maps the memory privately so the file will not be changed when you change the piddle. Be aware though that mmapping a 40Mb file without ReadOnly spends no virtual memory but with ReadOnly it does reserve 40Mb.

Example: Converting ASCII to raw

You have a whole slew of data files in ASCII from an experiment that you ran in your lab. You're still tweaking the analysis and plots, so you'd like if your data could load as fast as possible. Eventually you'll read the data into your scripts using readfraw, but the first thing you might do is create a script that converts all the data files to raw files:

Example: readfraw

Now that you've gotten your data into a raw file format, you can start working on your analysis scripts. If you scripts used rcols in the past, the reading portion of the script should go much, much faster now:

Example: Custom headers

In the first example, I allow writefraw to use the standard header file name, which would be file.bdat.hdr. However, I often measure time series that have identical length, so all of those header files are redundant. To fix that, I simply pass the Header option to the writefraw command. A modified script would look like this:

Example: Using mapfraw

Sometimes you'll want to use mapfraw rather than the read/write functions. In fact, the original author of the module doesn't use the read/write functions anymore, prefering to always use mapfraw. How would you go about doing this?

Assuming you've already saved your data into the raw format, the only change you would have to make to the script in example 2 would be to change the call to readfraw to mapfraw. That's it. You will probably see differences in performance, though I (David Mertens) couldn't tell you about them because I haven't played around with mapfraw much myself.

What if you eschew the use of writefraw and prefer to only use mapfraw? How would you save your data to a raw format? In that case, you would have to create a mapfraw piddle with the correct dimensions first using

mapfraw

The mapfraw command supports the following options (not all combinations make sense):

Dims, Datatype

If creating a new file or if you want to specify your own header data for the file, you can give an array reference and a scalar, respectively.

Creat

Create the file. Also writes out a header for the file.

Trunc

Set the file size. Automatically enabled with Creat. NOTE: This also clears the file to all zeroes.

ReadOnly

Disallow writing to the file.

Header

Specify the header file name.

maptextfraw

Memory map a text file (see the module docs also).

Note that this function maps the raw format so if you are using an operating system which does strange things to e.g. line delimiters upon reading a text file, you get the raw (binary) representation.

The file doesn't really need to be text but it is just mapped as one large binary chunk.

This function is just a convenience wrapper which firsts stats the file and sets the dimensions and datatype.

$pdl4 = maptextfraw("fname", {options}

The options other than Dims, Datatype of mapfraw are supported.

BUGS

Should be documented better. writefraw and readfraw should also have options (the author nowadays only uses mapfraw ;)

AUTHOR

Copyright (C) Tuomas J. Lukka 1997. All rights reserved. There is no warranty. You are allowed to redistribute this software / documentation under certain conditions. For details, see the file COPYING in the PDL distribution. If this file is separated from the PDL distribution, the copyright notice should be included in the file.